For press molding and bending work of high-strength parts and components of an automobile, an industrial machine and the like, a cold-rolled steel sheet used for such processing needs be excellent in both strength and workability. The recent years have seen a rising need, driven by a reduction of the weight of an automobile, to a cold-rolled steel sheet which has an even higher strength, and a TRIP steel sheet in particular is gaining an increased attention as a cold-rolled steel sheet which meets the need.
A TRIP steel sheet is a steel sheet in which an austenite structure remains present and which significantly elongates as residual austenite (γR) is induced to transform into martensite due to stress when processed and deformed at a temperature equal to or higher than the martensitic transformation start temperature (Ms point). Known as such are a few types, including for example a steel sheet whose matrix is polygonal ferrite and which contains residual austenite, a steel sheet whose matrix is tempered martensite and which contains residual austenite, a steel sheet whose matrix is bainitic ferrite and which contains residual austenite, a steel sheet whose matrix is bainite and which contains residual austenite (as that described in patent Document 1, for example), etc.
Of these, a steel sheet whose matrix contains bainitic ferrite and residual austenite is characterized in that it is easy to attain a high strength due to hard bainitic ferrite, it is easy to generate very fine residual austenite at the boundary of lath bainitic ferrite and such a morphological structure realizes excellent elongation. Further, there is an advantage related to manufacturing that such a steel sheet is easily produced through one thermal treatment (continuous annealing or plating).
However, even this steel sheet has a problem that as its strength increases, the workability decreases. To solve the problem, Patent Document 2 proposes a high-strength thin steel sheet in which one type or more from among Ni, Cu, Cr, Mo and Nb is added to a basic component composition for better hydrogen-resistant embrittlement, weldability and hole expanding capability. However, owing to the existence of bainitic ferrite to which an alloy element is indispensable and whose matrix has an extremely high dislocation density, a further improvement of ductility including total elongation is considered to be difficult. Meanwhile, it is desirable to reduce an alloy element from the perspectives of a cost, recycling, etc.                Patent Document 1: JP 01-159317, A        Patent Document 2: JP 2004-332100, A        